17,452 research outputs found
Potential of an ionic impurityin a large He cluster
This paper presents an analysis of the motion of an impurity ion in a
nanometer scale He cluster. Due to induction forces, ions are strongly
localized near the center of the cluster, with a root mean squared thermal
displacements of only a few \AA. The trapping potential is found to be nearly
harmonic, with a frequency of 2.3(1.0) GHz for a positive (negative) ion in a
He cluster of radius 5 nm. The anharmonicity is small and positive (energy
increases slightly faster than linear with quantum number). It is suggested
that by using frequency sweep microwave radiation, it should be possible to
drive the ion center of mass motion up to high quantum numbers, allowing the
study of the critical velocity as a function of cluster size.Comment: 14 pages, 0 figures, To be published in Molecular Physic
Quantum Hydrodynamic Model for the enhanced moments of Inertia of molecules in Helium Nanodroplets: Application to SF
The increase in moment of inertia of SF in helium nanodroplets is
calculated using the quantum hydrodynamic approach. This required an extension
of the numerical solution to the hydrodynamic equation to three explicit
dimensions. Based upon an expansion of the density in terms of the lowest four
Octahedral spherical harmonics, the predicted increase in moment of inertia is
, compared to an experimentally determined value of , i.e., 55% of the observed value. The difference is likely in at
least part due to lack of convergence with respect to the angular expansion,
but at present we do not have access to the full densities from which a higher
order expansion can be determined. The present results contradict those of Kwon
et al., J. Chem. Phys. {\bf 113}, 6469 (2000), who predicted that the
hydrodynamic theory predicted less than 10% of the observed increase in moment
of inertia.Comment: 10 pages, including 1 figur
Hot Electron Effects in the 2D Superconductor-Insulator Transition
The parallel magnetic field tuned two-dimensional superconductor-insulator
transition has been investigated in ultrathin films of amorphous Bi. The
resistance is found to be independent of temperature on both sides of the
transition below approximately 120 mK. Several observations suggest that this
regime is not intrinsically "metallic" but results from the failure of the
films' electrons to cool. The onset of this temperature-independent regime can
be moved to higher temperatures by either increasing the measuring current or
the level of electromagnetic noise. Temperature scaling is successful above 120
mK. Electric field scaling can be mapped onto temperature scaling by relating
the electric fields to elevated electron temperatures. These results cast doubt
on the existence of an intrinsic metallic regime and on the independent
determination of the correlation length and dynamical critical exponents
obtained by combining the results of electric field and temperature scaling.Comment: 4 pages, 4 figure
Measurement of the Neutral Weak Form Factors of the Proton
We have measured the parity-violating electroweak asymmetry in the elastic
scattering of polarized electrons from the proton. The kinematic point
(theta_lab = 12.3 degrees and Q^2=0.48 (GeV/c)^2) is chosen to provide
sensitivity, at a level that is of theoretical interest, to the strange
electric form factor G_E^s. The result, A=-14.5 +- 2.2 ppm, is consistent with
the electroweak Standard Model and no additional contributions from strange
quarks. In particular, the measurement implies G_E^s + 0.39G_M^s = 0.023 +-
0.034 (stat) +- 0.022 (syst) +- 0.026 (delta G_E^n), where the last uncertainty
arises from the estimated uncertainty in the neutron electric form factor.Comment: 10 pages, 4 figures, submitted to Phys. Rev. Let
Differing preferences of Antarctic soil nematodes for microbial prey
We tested the preferences of three nematode taxa, Geomonhystera villosa, Plectus spp. and Teratocephalus spp., extracted from moss at Signy Island in the Maritime Antarctic, for two microalgae, three microfungi and six heterotrophic bacteria, each also from soils at Signy Island. Choice test experiments on water agar medium, in which nematodes were enumerated in wells containing microbes at 24 and 48 h, indicated that there were differing preferences between nematodes for distinct prey. G. villosa was significantly attracted to the alga Chlorella cf. minutissima and the fungus Mortierella hyalina, and was more attracted to all algae and fungi than either of the other two nematodes. Both G. villosa and Teratocephalus spp. were attracted to an actinobacterium. Plectus spp. were significantly attracted to the alga Stichococcus bacillaris and bacteria with close taxonomic affinities to Arthrobacter, Pseudomonas and Polaromonas. Experiments using 0.5 μm diameter fluorescent beads indicated significantly increased ingestion by nematodes in the presence of each of these microbes compared with controls, except by Plectus spp. in the presence of S. bacillaris. We conclude that complex trophic interactions may occur in apparently simple Antarctic soil food webs
On integers for which the sum of divisors is the square of the squarefree core
We study integers n > 1 satisfying the relation σ(n) = γ(n) ² , where σ(n) and γ(n) are the sum of divisors and the product of distinct primes dividing n, respectively. We show that the only solution n with at most four distinct prime factors is n = 1782. We show that there is no solution which is fourth power free. We also show that the number of solutions up to x > 1 is at most x ⅟⁴⁺ᵉ for any ε > 0 and all x > xε. Further, call n primitive if no proper unitary divisor d of n satisfies σ(d) | γ(d) ² . We show that the number of primitive solutions to the equation up to x is less than xᵉ for x > xₑ
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